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J Dent Res 85(1):38-43, 2006
© 2006 International and American Associations for Dental Research

RESEARCH REPORT
Biomaterials & Bioengineering

Polymerization Kinetics of Pre-heated Composite

M. Daronch1,2, F.A. Rueggeberg2,*, M.F. De Goes1,3, and R. Giudici4

1 Department of Dental Materials, School of Dentistry, University of São Paulo, São Paulo, SP, Brazil;
2 Dental Materials Section, Department of Oral Rehabilitation, Medical College of Georgia, Augusta, GA 30912-1260, USA;
3 Department of Restorative Dentistry, Dental School of Piracicaba at UNICAMP, Piracicaba, SP, Brazil; and
4 Department of Chemical Engineering, Polytechnic School, University of São Paulo, São Paulo, SP, Brazil

* corresponding author, frueggeb{at}mcg.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Temperature affects the polymerization behavior of dimethacrylate-based materials. This study describes the influence of pre-polymerization temperature and exposure duration on polymerization kinetics of a commercial dental photo-activated composite at the top and at 2-mm depth. We used the temperature-controlled stage of a diamond-attenuated-total-reflectance unit to pre-set composite temperature between 3° and 60°C. Composite was light-exposed by a conventional quartz-tungsten-halogen curing unit for 5, 10, 20, or 40 sec. Real-time conversion, maximum conversion rate (Rpmax), time to achieve Rpmax, and conversion at Rpmax were calculated from infrared spectra. Composite pre-warming enhanced maximal polymerization rate and overall monomer conversion (top significantly greater than 2 mm). Time when Rpmax occurred did not change with temperature, but occurred sooner at the top than at 2-mm depth. Conversion at Rpmax increased with temperature, allowing more of the reaction to occur prior to vitrification than at room temperature.

KEY WORDS: resin composite • kinetics • polymerization rate • monomer conversion • curing temperature


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS & METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Photo-activated polymerization of dimethacrylate-based materials is based on free-radical formation and leads to a strong, cross-linked network. The reaction kinetics of these multifunctional monomers used for dental restorations is a multifaceted process exhibiting complex features such as autoacceleration, autodeceleration, limited final conversion, cyclization, and radical trapping. This complex behavior arises from the decrease in mobility of reaction media by network formation as polymerization proceeds, leading to the onset of autoacceleration from the very beginning of polymerization (Andrzejewska, 2001). This phenomenon, also called the gel effect, corresponds to a sudden increase in reaction rate (Rp), despite the monomer’s being consumed. It is generally accepted that autoacceleration occurs due to changes in the termination mechanism that becomes diffusion-limited, causing a reduction of the termination rate constant (kt), and a consequent increase in the concentration of free-radicals. Polymerization rate reaches its maximum value (Rpmax), and then the reaction proceeds with decreasing rate (autodeceleration), as propagation also becomes diffusion-controlled. Decreased Rp during autodeceleration is attributed to reduced mobility of both monomer and unreacted pendant double-bonds, and decreasing dissociation efficiency of photo-initiators in the viscous media (cage effect) (Cook et al., 1997; Lecamp et al., 1997; Lovell et al., 1999, 2001a; Nie et al., 1999; Andrzejewska, 2001; Sideridou et al., 2002). Upon continued reaction and crosslinking, mobility is reduced further, and the system becomes even more entangled and viscous until the reaction stops due to polymer vitrification. The onset of vitrification occurs when the increasing glass transition temperature (Tg) of the reaction mixture reaches the polymerization temperature. The reaction rate will undergo a significant decrease after vitrification, and the reaction becomes very slow as it is controlled by the diffusion of the reactive species. The diffusion-controlled effect, which produces a slow-down of the polymerization process, will also determine the final degree of conversion (Odian, 1982).

The Rpmax occurs close to the point where reaction diffusion (in which radical pairs meet by a series of random chain propagation steps) starts to dominate over the translational/segmental diffusion termination mechanism (Cook, 1992). The temperature at which polymerization occurs affects the conversion process and polymer properties (Bajaj et al., 1977; Bausch et al., 1981; Cook, 1992; Cook et al., 1997). Increased temperature enhances both radical and monomer mobility, resulting in higher overall conversion and Rpmax (Cook, 1992; Lovell et al., 2001b).

Temperature has been shown to have a significant effect on final conversion values of commercial dental resin composites. Pre-heating a microhybrid composite to 54° or 60°C produces higher conversion, allowing for reduced exposure duration up to 75%, while still attaining conversion values similar to those achieved with the longer exposure recommended for room temperature (Daronch et al., 2005). Attainment of high conversion values by short exposure times implies a more rapid photopolymerization at elevated temperatures. Such high reaction rates may result in elevated stress formation and hasten development of the vitrification point, proving detrimental to the integrity of the resin/tooth interfacial bond (Uno and Asmussen, 1991; Sakaguchi and Berge, 1998). However, increased conversion of composite from being pre-heated may also result in enhanced restoration properties (Lovell et al., 2001b). Thus, it is important that one evaluate the kinetics of composite curing within a range of clinically relevant temperatures to understand factors that may affect the eventual success of a restoration.

This study investigated kinetic polymerization parameters at the top (irradiated) and 2-mm depth of a commercial composite as a function of pre-polymerization temperature and light-exposure duration. We tested the hypotheses that temperature increase (1) would increase maximum conversion rate (Rpmax) and final conversion, (2) would reduce the time to achieve maximum conversion rate (tmax), (3) would increase conversion attained at Rpmax, and that (4), at a given temperature, Rpmax at the top surface would be greater than at the bottom depth, but that tmax would occur sooner at the top than at 2-mm depth.


   MATERIALS & METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Specimen Preparation
A commercial, photo-activated hybrid resin composite was used (Esthet·X, shade A2, lot # 030221; Dentsply/Caulk, Milford, DE, USA). Detailed experimental set-up and data collection have been previously described (Daronch et al., 2005). In summary, composite was placed into a 2-mm-high brass ring on the temperature-controlled stage of a diamond-attenuated-total-reflectance unit (Heatable Golden Gate ATR, MKII; SPECAC Inc., Smyrna, GA, USA). Composite pre-cure temperatures were 3°, 10°, 20°, 22° (control), 25°, 27°, 30°, 40°, 54°, and 60°C. The latter two values are similar to those used in a commercial composite pre-heating device (CalsetTM; AdDent Inc., Danbury, CT, USA). The 3°C value represented refrigerator storage temperature. The upper composite surface was exposed with a conventional quartz-tungsten-halogen light-curing unit (Optilux 501; Demetron/Kerr Co., Orange, CA, USA) for 5, 10, 20 (control: manufacturer-recommended exposure for a 2-mm-thick increment), or 40 sec. We obtained spectra representing top surface values by pressing composite into a thin film ({approx} 150 µm) on the pre-heated stage and photocuring as above. Curing unit irradiance was 630 mW/cm2 between 350 and 600 nm, measured with a laboratory-grade spectral radiometer (DAS 2100; Labsphere, N. Sutton, NH, USA). Infrared spectra between 1680 and 1550 cm–1 were collected at both 0-mm and 2-mm composite depths for 305 sec at a rate of 1 scan/sec, in a Fourier transform infrared spectrometer (FTS-40; Digilab/BioRad, Cambridge, MA, USA). Monomer conversion was determined by standard methods that utilize changes in the ratios of aliphatic-to-aromatic C=C absorption peaks in the uncured and cured states (Ruyter and Øysæd, 1982; Ferracane and Greener, 1984; Rueggeberg et al., 1990).

Data Analysis
Calculation of polymerization kinetics followed methods previously described (Rueggeberg et al., 1999). Infrared spectra yielded information on conversion rate (first derivative of real-time conversion), maximum conversion rate (peak value in 1st derivative, Rpmax), time to achieve Rpmax (tmax), and conversion at maximum rate.

Statistical Analyses
Two-way analysis of variance (ANOVA)/Dunnett’s two-tailed post hoc t test compared conversion values with those attained at 22°C for a 20-second exposure (control). One-way ANOVAs were also performed with the independent variable being temperature or depth. Comparisons were performed for each kinetic parameter. Individual unpaired, one-tailed Student’s t tests were performed at each temperature between top and bottom parameter values. One-way ANOVA/Tukey’s test detected differences in kinetic parameters with respect to exposure duration at a given temperature and surface. All statistical testing was performed at a pre-set {alpha} of 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Overall Observations
Control Rpmax (22°C/20 sec) at the top was 13.8%/sec, reached after only 2.0 sec of light exposure; whereas, at 2-mm depth, Rpmax was 7.3%/sec, reached at 2.8 sec (TableGo). The most extreme heated condition (60°C/5 sec) exhibited Rpmax of 24.2%/sec (top), occurring after 2.0 sec, and 9.8%/sec (2-mm depth), reached after 3.6 sec. Control monomer conversion when composite reached Rpmax was 16.2% (top) and 11.2% (2 mm), and 28.2% (top) and 19.5% (2 mm) when pre-heated (60°C/5 sec). Composite cured at 3° and 10°C exhibited lower Rpmax and conversion at Rpmax than did the control.


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  		Table. Mean (SD) of Kinetic Parameters for Composite Polymerization: Effects of Temperature, Exposure Duration, and Surfacea
 
Real-time Conversion
At 20-second exposure, conversion significantly increased with temperatures above control (Fig. 1Go). Most of the reaction occurred within the first 10 sec. Top surface conversion was always significantly greater than at 2-mm depth at any given pre-heated temperature. Final conversion at 300 sec increased approximately 35% between control and extreme pre-heated conditions (54 and 60°C).



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  		Figure 1. Conversion with time at the top surface and at 2-mm depth with 20-second exposure duration at pre-selected composite temperatures. n = 5 specimens per experimental group. Coefficient of variation ranged from 0.3 to 3.0%.

 
Maximum Conversion Rate
Rpmax increased with temperature, and statistical differences were detected between values at 3°, 30°, 40°, 54°, 60°C, and control (22°C) for both composite surfaces (TableGo, Figs. 2A–2DGo). Top surface values exceeded those of the bottom at each temperature. Pre-cure composite temperature and Rpmax followed an Arrhenius-type relationship. We determined the activation energy (Ea) at Rpmax with a 20-second exposure between 3° and 60°C by plotting the natural log of conversion rate vs. inverse temperature (Lovell et al., 1999). The slope of the linear regression represented Ea: 15.6 kJ/mol (Fig. 2EGo).



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  		Figure 2. Conversion rate with time at the top surface and at 2-mm depth, at selected temperatures and exposure durations: (A) 40 sec, (B) 20 sec, (C) 10 sec, and (D) 5 sec, (E) natural log of Rpmax as a function of inverse temperature. A linear fit to the data was used to determine the activation energy (15.6 kJ/mol). (F) Natural log of conversion rate as a function of inverse temperature and monomer conversion at the top surface and (G) at 2-mm depth. For A–D, data at each one-second time point represent the mean of 5 replications; coefficient of variation ranged from 3.1 to 17.6%.

 
Time into Exposure of Rpmax (tmax)
The tmax ranged from 1.8 sec (40°, 54°, and 60°C/top surface) to 4.4 sec (3°C/2-mm depth), occurring significantly sooner at the top than at 2-mm depth for all temperatures and exposure durations (TableGo). Temperature increase diminished tmax most noticeably at 2-mm depths. This reduction, however, was not statistically significant when compared with those of the control.

Conversion at Rpmax
Conversion at Rpmax increased with temperature, being significantly greater on the top than at 2-mm depths for all temperatures and exposure durations (TableGo, Figs. 3A–3DGo). Conversion at Rpmax was 8.3% at 3°C, as opposed to 28.2% at 60°C (top).



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  		Figure 3. Conversion rate as a function of monomer conversion obtained at the top surface and at 2-mm depth for various temperatures and exposure durations: (A) 40 sec, (B) 20 sec, (C) 10 sec, and (D) 5 sec. Each datapoint represents mean of 5 replications; coefficient of variation ranged from 6.0 to 36.4%.

 
Effect of Exposure Duration on Kinetic Parameters
For most test conditions, exposure duration did not have a significant effect on kinetic parameters when compared at a specific temperature. However, at 2-mm depth, tmax at 10 sec (40°C) or 5 sec of exposure (54° and 60°C) occurred later than at 20 sec of exposure. Also, at 2-mm depth, 10- and 5-second exposures produced significantly lower Rpmax than did 20-second exposures at 22° and at 40°C.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS & METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study demonstrated the strong influence of composite temperature on polymerization kinetics of a dental restorative composite. The first and third hypotheses were validated. Temperature increase dramatically improved molecular mobility, increasing collision frequency of reactive species, and allowing for further monomer conversion before the onset of autodeceleration. Increase in Rpmax with temperature was expected, and was in agreement with published reports (Cook, 1992; Watts, 1992; Nie et al., 1999; Lovell et al., 2001b; Trujillo et al., 2004). Increased molecular mobility allowed propagation to continue for a longer time without becoming diffusion-controlled, increasing conversion at Rpmax. Furthermore, temperature rise improved the segmental mobility of the polymer chain, postponing the change in the termination mechanism from one diffusion-controlled to one controlled by reaction diffusion (Cook, 1992; Andrzejewska, 2001; Scott et al., 2002). In this way, more monomer could convert into polymer prior to vitrification. The glass transition temperature of the reacting mixture increases as monomer conversion rises, so greater final limiting conversion can be achieved at higher polymerization temperatures.

The kinetic constants for dimethacrylate polymerization exhibited an Arrhenius-type behavior (e.g., large increase in reaction rate with small temperature increase) (Billmeyer, 1984; Lovell et al., 2001b). Apparent activation energy at Rpmax (15.6 kJ/mol) was similar to that reported by others using dimethacrylates (15.5 kJ/mol; Lecamp et al., 1997), diacrylates (13.5 KJ/mol; Scherzer and Decker, 2000), and methylmethacrylate (17.1 kJ/mol; Odian, 1982), and was within the range expected for reaction-diffusion-controlled termination and for translational diffusion (Scott et al., 2002). It should be noted that the Arrhenius plot for the rate data taken at constant conversion presents a more complex behavior (Figs. 2F, 2GGo). Quite similar trends were observed by Cook (1992) and Scott et al.(2002). At lower temperatures and higher conversions, the polymerization rate becomes considerably slower as the propagation, termination, and initiation steps become diffusion-controlled, due to a decrease in molecular mobility.

Composite reacts quickly once light exposure is initiated (Fig. 2Go). The tmax occurred before 5 sec of light exposure at 2-mm depth, and even sooner—under 3 sec—at the top surface. At a given surface, as temperature increased, tmax slightly decreased. This change, however, was not statistically significant. Thus, the second hypothesis was rejected. This finding is in agreement with those of others (Maffezzoli et al., 1994; Nie et al., 1998). However, other studies also found that tmax can increase or remain almost constant (Cook, 1992) or decrease with reduction in temperature (Cook, 1993). Thus, this effect seems to be dependent on the system nature and on the experimental conditions used. Final conversion increased with temperature, and its dependence on exposure duration has been established (Daronch et al., 2005). The effect of exposure duration on kinetic parameters is clinically relevant in the context of the use of short exposures (5 or 10 sec) when composite is pre-heated. At a given temperature and surface, Rpmax and tmax should not be dependent on exposure duration. As long as the duration of light exposure exceeds the occurrence of Rpmax, kinetic parameters would not be affected. Exposures shorter than this critical time would result in lower conversion, yielding poorer properties. Some commercial light-curing units advocate exposure durations of only 1 to 3 sec, possibly not fulfilling this requirement. In the present study, we measured conversion on the top surface and at 2-mm depth, which is the maximal accepted thickness for placement of composites (Rueggeberg et al., 1994). We achieved equivalent cure at 2 mm in less exposure time with increased temperature. It should be noted, though, that while elevated cure temperatures can facilitate reduced irradiation time for the same conversion, the depth of cure will probably be less, based on the logarithmic relationship between the depth of cure and irradiation dose (Cook and Standish, 1983).

The fourth hypothesis was upheld. At a given temperature, Rpmax was greater and tmax occurred sooner at the top surface than at the bottom. These results are based on factors affecting photo-initiation and the ability of light to penetrate: Radiation is attenuated and initiation is delayed at greater depths (Cook and Standish, 1983). Therefore, irradiance at the bottom surface will always be lower than at the top. Temperature can affect the reaction rate by influencing rates of propagation and termination. However, the increase in cure temperature has a minimal effect on the decomposition rate of photoinitiators, since it is light rather than heat that leads to radical formation (Trujillo et al., 2004). The effect of low temperature on composite polymerization was significant. When composite was polymerized at 3°C, system viscosity was apparently so great that Rpmax occurred at less than 10% conversion, and final conversion was limited to values below 35%. These results stress that refrigerator-stored composite should be allowed to reach room temperature prior to clinical use.

The results of this study demonstrate that pre-warming composite enhances conversion without hastening the time at which maximum cure rate occurs at the top and at 2-mm depth. This enhancement is probably attained by increased molecular mobility resulting from the temperature increase and, thus, the postponement of diffusion-controlled propagation, reaction-diffusion-controlled termination, and autodeceleration, thereby allowing the system to reach higher limiting conversions before vitrification. As a result, a more highly crosslinked polymer network and improved mechanical and physical properties may be anticipated from composites when they are pre-heated to temperatures above that of the room.


   ACKNOWLEDGMENTS
 
The assistance and dedication of Ms. Linda Moss and the talents of Mr. Don Mettenburg are greatly appreciated. This research was supported by CAPES Brazil (grant 03/48031) and by The Medical College of Georgia School of Dentistry. The authors thank Dentsply/Caulk for supply of composite. This paper is in partial fulfillment of requirements for the PhD degree for Dr. Márcia Daronch, University of São Paulo, São Paulo, SP, Brazil.

Received June 7, 2004; Last revision August 29, 2005; Accepted September 15, 2005


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